1. Field of the Invention
The present general inventive concept relates to a bidirectional equalizer, and more particularly, to a bidirectional equalizer, which can correctly select an output with the lowest error probability among outputs of a forward equalizer and a reverse equalizer without channel estimation, and an equalization method using the bidirectional equalizer.
2. Description of the Related Art
In general, a major factor degrading the performance of a high speed digital communication system is inter-symbol interference. The inter-symbol interference is caused by linear channel distortion, a multipath channel, abnormal frequency characteristics, and so on. Many studies have been conducted to reduce such inter-symbol interference caused by channel distortion, and channel equalizers, which can reduce bit detection error by compensating for a received signal passing through a distorted channel using a particular algorithm, have been developed.
Conventional channel equalizers use a method in which a transmitter transmits a training sequence, which is recognizable by a receiver, to the receiver for a predetermined period of time, and the receiver compares a received signal with an original signal and estimates the level of distortion of a channel. Also, the conventional channel equalizers perform equalization by continuously controlling tap coefficients using a least mean square (LMS) algorithm that is relatively simple to implement.
However, since the conventional channel equalizers are unidirectional, it is difficult for the conventional channel equalizers to effectively eliminate interference between pre-ghost symbols, and also end up amplifying a high frequency component while removing inter-symbol interference, resulting in increasing noise in a received signal. To solve the above listed problems, bidirectional equalizers have been suggested.
There are many types of bidirectional equalizers. Among them, bidirectional arbitrated decision-feedback (BAD) equalizers stand out for their high performance. For more information, see the study by A. C. Singer, U. Madhow, C. S. McGahey, and J. K. Nelson, entitled “Bidirectional Arbitrated Decision-Feedback Equalization”, in IEEE Trans. Commun., vol. 53, no 2. 214-218, February 2005.
Conventional BAD equalizers use an algorithm that performs by: generating two candidate data sequences using a forward equalizer and a reverse equalizer; passing the generated candidate data sequences through an estimated channel and reconstructing a received signal; comparing the reconstructed received signal with an original received signal; and outputting an output with the lowest error probability based on the comparison result. Meanwhile, the conventional BAD equalizers use the impulse response characteristics of a training sequence to estimate a channel.
However, it is difficult for the conventional BAD equalizers to accurately estimate a channel due to a limitation in the length of a training sequence, and even more difficult to accurately estimate a channel when using training sequences received at regular intervals in a dynamic environment where channel characteristics change frequently. Accordingly, the conventional BAD equalizers have a high risk of error during the reconstruction of a received signal passed through an estimated channel.